fix: snapshot proxy before save — fixes 422 Unprocessable Entity

Frontend/src/lib/save-sync.svelte.ts

@@ -26,12 +26,17 @@ let lastSave: string | null = null;
 let loaded = false;
 let saveInterval: ReturnType<typeof setInterval> | null = null;
 
+// Snapshot the $state proxy into a plain object for serialization
+function snapshotState(): Record<string, unknown> {
+	return JSON.parse(JSON.stringify(game.state));
+}
+
 export async function saveToServer() {
 	if (!authStore.user || !game.ready) return;
 	const result = await apiRequest('/save', {
 		method: 'POST',
 		body: JSON.stringify({
-			gameState: game.state,
+			gameState: snapshotState(),
 			playTimeSeconds: game.playSeconds,
 		}),
 	});
@@ -103,7 +108,7 @@ export function setupVisibilitySync() {
 	window.addEventListener('beforeunload', () => {
 		if (!authStore.user || !game.ready) return;
 		const payload = JSON.stringify({
-			gameState: game.state,
+			gameState: snapshotState(),
 			playTimeSeconds: game.playSeconds,
 		});
 		fetch(`${BACKEND_URL}/api/save`, {